1. Field of the Invention
The present invention is directed to a rotary pump including an impeller, a shaft and a rotor and having a permanent magnetic drive, and more particularly, to the lubrication of the plain bearing assembly which supports the shaft.
2. Description of the Prior Art
Rotary pumps including an impeller integrally formed with or fixedly attached to a rotary shaft are known in the prior art, for example, as disclosed in U.S Pat. No. 4,120,618, which is incorporated by reference. A rotor, shaft and impeller are integrally formed, and a plurality of permanent magnets are disposed circumferentially about the rotor, within the cylindrical section of an isolation shell. A corresponding plurality of permanent magnets are disposed on the interior of an outer rotor, across the shell from the first set of magnets. The magnets are disposed with opposite poles opposing each other across the cylindrical section of the shell. Therefore, rotation of the outer rotor causes corresponding rotation of the rotor, shaft and impeller. The impeller is part of a normal rotary pump which is well known to the person skilled in the art. In normal operation, the fluid to be pumped is present substantially throughout the entire interior of the pump, including the region occupied by the bearing assembly which supports the rotary shaft.
In rotary pumps of this type, since the fluid to be pumped is present throughout the interior pump, plain bearings must be used to support the rotor. In addition, by apparent reasons, the plain bearings cannot be oil-lubricated, but are lubricated by the pumped medium itself. However, the rotary pumps are very often used for pumping fluids such as acids, leaches, solving means and highly reactive fluids which explode when in contact with oxygen, none of which have favourable lubricating properties. Thus, plain bearing assemblies made of a hard ceramic material have been developed which last very long despite the unfavourable conditions.
For proper lubrication of the bearing assembly, it is necessary for the interior of the pump to be maintained substantially filled with the pumped medium at all times during operation. During operation, a secondary flow of fluid within the pump circulates due to pressure differences at the start and the end respectively of the secondary flow, and this flow lubricates and cools the bearing assembly. However, dry running of the pump may occur, after the system which includes the pump is drained, or the pump or system may develop a leak before or during operation which is not immediately detected. Thus, the secondary flow of fluid medium around and through the bearing assembly may be insufficient to provide sufficient lubrication and cooling. Accordingly, a large quantity of heat will be generated between the bearing and the shaft, eventually resulting in destruction of the bearing assembly. In addition, the pump may be installed incorrectly, for example, by reversing the electrical connections during installation, such that the impeller is turned in the wrong direction. Once again, the secondary flow of the fluid medium may be insufficient to remove enough heat from the bearing assembly to prevent damage.